Texture-induced anisotropic phase transformation in a NiTi shape memory alloy

Abstract

Texture-induced anisotropic phase transformation of a rolled NiTi sheet was investigated via uniaxial tension and cyclic loading using in situ laboratory X-ray diffraction. The tensile tests were carried out along rolling direction (RD) and transverse direction (TD), which respectively have the highest and lowest phase transformation plateau strain. The initial preferred orientations of the rolled sheet (single austenite phase) concentrate on the ζ and γ fibers, i.e. <110>∥ND and <111>∥ND (normal direction). For loading along RD, six favorably oriented martensite variants form and lead to the formation of the new texture component {111}<112>, whereas along TD, three favorably oriented martensite variants form, making the initial {110}<110> texture component stronger. Besides, the Lüders band can be clearly observed in the first cycle of cyclic loading, during which the favorably oriented martensite variants are firstly induced, followed by the less-favorably oriented martensite variants. The martensite transformation continues after the phase transformation plateau via the propagation of the favorably and less-favorably oriented martensite variants. At high cycles, Lüders band cannot be observed due to remained traces of martensite and grain refinement.